When Two Pieces of Glass Are Pressed Together in Air

Multiple Choice

When two pieces of glass are pressed together in air and illuminated with coherent light, the place where they are in contact will exhibit an appearance correctly described in one of the responses to the preceding question. The reason behind the correct answer is that

A) the actual physical path difference between waves striking the two surfaces in contact is very close to zero.
B) the phase difference between waves reflected from the two surfaces is 180º.
C) Both of the first two responses are necessary to obtain the correct answer.
D) Though both of the first two responses state facts correctly, neither is relevant in explaining the correct answer to the previous question.